This invention relates to a heat-pump type air conditioning apparatus in which heating or cooling operation can be adequately switched from normal, full-capacity operation to reduced-capacity operation.
A heat-pump type air conditioning apparatus generally comprises a compressor, heat exchangers located on indoor and outdoor sides of the system (hereinafter called indoor and outdoor heat exchangers, respectively), a throttling member, and fans located on indoor and outdoor sides of the system (hereinafter called indoor and outdoor fans, respectively).
In the heating operation, for example, of the heat-pump type air conditioning apparatus, a refrigerant discharged from the compressor towards a four-way valve circulates through the indoor heat exchanger acting as a condenser, a capillary tube acting as an expansion member, the outdoor heat exchanger acting as an evaporator, the four-way valve, and then returns to the compressor. The indoor and outdoor fans are provided for the indoor and outdoor heat exchangers, respectively, which act as the condensor and evaporator, to enhance heat exchanging effects.
During the heating operation, since the temperature difference between indoors and outdoors generally changes relatively abruptly, refrigerant compressing load on the compressor also widely changes. Under the circumstances, when a voltage of a power supply source adapted to operate the compressor lowers, the output of the electric motor for driving the compressor also lowers, and particularly, when the refrigerant compressing load is considerably high, there may occur in an extreme case where the compressor comes to stop.
In order to obviate this defect, it was necessary to control the load reduction of the compressor by stopping the outdoor fan or controlling the output of the compressor. Control of load reduction was carried out by stopping the outdoor fan and then controlling the opening and closing of an electromagnetic valve incorporated into an unloading or return circuit which acts so as to return a portion of the refrigerant from the pressure chamber of the compressor to the suction side thereof. The timing for deenergizing the outdoor fan driving motor and for opening the electromagnetic valve in the return or unloading circuit were determined by either one of the following two methods. The method for deenergizing the outdoor fan motor was carried out by determining, as a control reference, a time at which the load applied on the compressor exceeded a predetermined value. In other words, discharge pressure of the refrigerant from the compressor or condensing temperature of the refrigerant at the indoor heat exchanger was measured. When the measured value exceeded a predetermined value, the outdoor fan driving motor was deenergized.
The other method for controlling the timing of opening the electromagnetic valve was carried out by determining the control reference at a time when the magnitude of the voltage of the power supply source or the magnitude of the current flowing through the compressor driving motor reached a predetermined value. Thus, the time was determined by the condition of the power source.
In the heating operation of a conventional heat-pump type air conditioning apparatus, the heating operation from the normal operation to reduced-capacity operation was switched by taking into consideration, as a control reference, one of either the load condition of the compressor or the power source condition of the compressor.
In a case where the heating operation is switched from the normal operation to the reduced-capacity operation solely on the basis of the load condition of the compressor, when the detected condensing temperature of the refrigerant exceeds a temperature predetermined for switching the heating operation, the outdoor fan driving motor is stopped or a portion of the refrigerant is returned through the unloading circuit. However, in this method, the predetermined temperature will have to be preset within a low temperature range, for example, between 45.degree.-55.degree. C. in view of a case where the voltage of the power supply source lowers to about 85% of the rated voltage. The low predetermined temperature hinders the condensing temperature at the indoor heat exchanger from rising and it has been impossible to attain sufficient heating capability.
In addition, when a commercial household AC source is utilized as a power source, the starting current has to be limited below 45A in accordance with the "Electric Appliance and Material Control Law", so that it becomes necessary to use a motor for driving the compressor with relatively small capacity or to use a reactor for limiting the starting current. For this reason, the output of the motor falls off sharply in a case where the voltage of the power source is low, and hence, it was necessary to determine the predetermined temperature to be in a considerably low temperature range of 45.degree.-55.degree. C.
On the other hand, where the heating operation is switched on the basis of the condition of the supply power source, the heating operation is switched from the normal operation to the reduced-capacity operation at a time when the voltage of the power source falls below a predetermined value or the current from the power source exceeds a predetermined value. In this case, when the voltage is considerably high, the output of the compressor becomes too large and the refrigerant discharge pressure exceeds a suitable pressure, thus lowering the durability of the compressor.
Regarding the cooling operation of the heat-pump type air conditioning apparatus of the type described above, it will easily be understood that the refrigerant from the compressor flows through the four-way valve and circulates in a direction opposite to that described in connection with the heating operation. In the cooling operation, when the load of the indoor or outdoor heat exchanger reduces, suction and discharge pressures of the compessor lower and the load on the compressor also lowers. The evaporation temperature of the indoor heat exchanger lowers in response to the lowering of the suction pressure, so that under an extreme condition frost may be formed on the indoor heat exchanger now acting as an evaporator. Moreover, when the load on the indoor heat exchanger lowers, the refrigerant under low pressure returns to the compressor without being completely evaporated, which results in a significant loss of compressor operation.